Gas turbine engines include compressor rotors including a plurality of rotating compressor blades. Minimizing the leakage of air, such as between tips of rotating blades and a casing of the gas turbine engine increases the efficiency of the gas turbine engine as the leakage of air over the tips of the blades can cause aerodynamic efficiency losses. To minimize this, the gap at tips of the blades is set small and at certain conditions, the blade tips may rub against and engage an abradable seal on the casing of the gas turbine. The abradability of the seal material prevents damage to the blades while the seal material itself wears to generate an optimized mating surface and thus reduce the leakage of air.
Abradable seals have also been used in turbines to reduce the gap between a rotor and a vane. Thermally sprayed abradable seals have been used in gas turbine engines since the late 1960s. The seals have been made as coatings from composite materials that derive their abradability from the use of low shear strength materials or from a porous, friable coating.
Rotor seal coatings running against the tips of cantilevered vanes have, nevertheless, room for improvement. The coating should not be too thermally conductive, such as an alumina coating. This could cause thermal expansion induced runaway events. Use of a more insulative coating such as zirconia could spall during deep or high rate rub interactions with the vanes. Both situations can result in having to establish more open clearances between the rotor shaft and the vane tips.
In the past, cantilevered vane rubs are typically limited to less than 2 mils (50.4 microns) and have less than full circumference contact due to the risks of high rub forces, coating spallation or a thermal runaway event where the heat from the rub causes thermal expansion of the rotor. The rotor, when heated sufficiently, can grow out to interfere with the vanes. The result can be a burn through causing holes in the rotating shaft, which can cause subsequent unscheduled engine removal.
A need exists for a coating that can prevent the rotor from being damaged during runaway events while still allowing the vanes to maintain acceptable sealing gap dimensions.